CROWDS WILL FLOCK to the Potomac this weekend to watch the President's Cup speedboat regatta. It's East Potomac Park's annual moment in the sun, but the flat, grassy stretch of parkland just below the Tidal Basin won't be at a loss for visitors all summer long.

While East Potomac Park is one of the business recreational facilities in the area, local residents might be surprised to know it also is considered one of the great man-made "island" parks in the world.

With sailboats and big motor yachts plying the channels on either side and jets thundering overhead on their way to and from National Airport, the park is an inner-city haven where people can get away from it all without leaving town.

On late summer afternoons bicyclists race around the outer road of the park known as the "speedway" while families spread their picnic blankets along the water's edge. Youngsters toss frisbees while others drop finishing lines into the river.

Golfers play East Potomac's 36 holes at 1950 [WORD ILLEGIBLE] at the southern tip of the park - Hains Point - couples stroll along the seawall; at the north end joggers turn up Buckeye Drive past chatting tennis players awaiting their turns on the courts.

For all its usefulness, East Potomac Park began as an accident. Shortly after the Civil War the Potomac had silted to the point it was beginning to obstruct river traffic, at that time vital to the commercial survival of Georgetown, the Chesapeake and Ohio Canal and the infant city of Washington.

Dredging operations began in 1870, with $50,000 appropriated by Congress. Those efforts were wiped out by the 1877 flood and Georgetown businessmen obtained their own "mud machine" and set to work.

In 1882 Congress again appropriated funds for dredging and placed Maj. Peter C. Hains of the Army Corps of Engineers in charge. Hains, for whom the point was named, was an 1861 West Point graduate and had fought in the Civil War.

Hains' major problem was in disposing of the tons of dredge spoil. He solved his dilemma by creating what would become East Potomac Park.

Hains' plan called for dredging both the Potomac and its Washington Channel and piling the dredge spoil onto the mud flats in between.

The mud flats, known then as the Potomac Flats, ranged from 17th Street below the White House west to Lincoln Memorial and east to the Bureau of Engraving and Printing. These flats were a health hazard because they were a breeding ground for malaria-carrying mosquitos. They were also the reason the city of Washington was known in those days as the "Cinderella of the Swamps."

Around this mountain of dredge Hains planned to throw up a stone sea wall to hold the man-made island in place. The plan also called for excavation of a tidal basin and construction of two new bridges across the Potomac.

The dredging began late in 1882. The mud was loaded onto scows and carried to a point near the flats, transferred to railroad cars and dumped onto the flats.

In 1889 a severe flood again undid much of the dredging. Flood waters rose along Pennsylvania Avenue and flowed down to the Anacostia River near the War College by way of Canal Street. Thrill-seekers boarded the one-horse streetcars operating on Pennsylvania Avenue (16 tickets for 25 cents), then had to stand on the seats to avoid getting wet.

After the flood, work began at once redredging the Tidal Basin. When it was completed, automatic flood gates were placed on either side of the basin the Georgetown Channel and be discharged into the Washington Channel as the tide went out.

This phenomenon has occurred twice a day ever since and ingeniously serves to flush the harbor and prevent stagnation along the city waterfront.

With the mud piled high on the flats, work began on the sea wall. In 1890 a moat was dug around the fill area and woven mats were placed along the bottom of the moat. Rocks and heavy fill were pushed in on top and willows were planted at Water's edge to hold the soil.

In 1891, after nine years of supervising the work, Hains was transferred. Two years later the land once known as the Potomac Flats was officially declared reclaimed.

In 1895 Congress tried to pass a bill making the 327 acres of reclaimed land a public park. It was defeated by those who wanted to sell the land to pay for the cost of reclaimation.

With the help of a local banker, Charles Carroll Glover, the public park proposal was revived and passed in 1897. Before the vote Glover visited the White House to make sure President Cleveland was receptive to the measure. He told the President it was "certainly gratifying to know and to feel that wonderful stretch of land is to become the great national park of the county."

Cleveland was surprised by the remark, since he wanted to make the area into a truck garden for raising vegetables. Glover, dumfounded, spent the next hour dissuading him.

During the early years after the turn of the century the pile of moist river bottom, which had long since been leveled off and dried out, began to produce a heavy cover of undergrowth. Men in hunting togs with guns and dogs became a familiar sight.

In 1912, at the suggestion of Mrs. Taft, wife of the President, the scenery was enhanced by planting 1,8000 cherry trees around the perimeter.

The following year the land was transferred to the Office of Public Buildings and Grounds for development as a park. Work soon began on a road around the outskirts which became known as the "Speedway."

The name "Speedway," a misnomer since speeding always has been forbidden, derived from the late 1800s, when there really was a speedway around the Washington Monument grounds.

But the time the new Speedway was completed in 1916 the world was at war; Boy Scouts cleared some of the land for a war garden. In the same year a plan was submitted for postwar development of the park as a recreation center.

Planners were aware of the steadily rising population of the nation's capital, which had jumped from 75,080 in 1860 to 331,059 in 1910.

A new plan was drawn up calling for golf tennis , swimming, polo, football and croquet facilities to be built on the park site, together with a 40,000-seat baseball stadium. But before the recreation park could take shape precedence went to the war effort. By 1918 the Boy Scout's garden had become a truck farm, 41 wooden barracks were erected on the Georgetown Channel side of the park to house Army troops and 57 others appeared on the Washington Channel side for enlisted clerks on duty at the War Department.

When the war was over construction began on golf course and picnic grounds. In July, 1920, the first nine-hole golf course was opened to the public; a second nine followed two years later. Tennis courts and camping facilities were provided and in 1925 a public swimming beach was opened on the rim of the Tidal Basin. The Bureau of Fisheries stocked the Tidal Basin with black bass.

In 1930 battery powered "speedboats" raced on the Tidal Basin, but only for two years before they were replaced by pedal boats. the speedboats, which were getting faster and more powerful year by year, moved to the Potomac, where the President's Cup Regatta soon became an annual event.

The park has prospered over the years.

Today the 36-hole golf course caters to as many as 900 players on a Saturday or Sunday. Prices have gone up since 1920, when you could play nine holes for 15 cents or for a quarter, but the current $1.50 for nine on a weekday and $2 on weekends is still unbeatable.

The 21 tennis courts, eight lighted for night play, are in constant use, including five which are covered with a nylon "bubble" during the winter for indoor play.

Emerging out of the mud flats of the Potomac, East Potomac Park has developed into one of the most picturesque urban recreational parks in the world. It has survived the technological changes of the 20th Century - from surreys to automobiles, horses to bicycles and wooden sailing ships to power craft.